I haven't had to personally "rebuild" a Unity server but I'd think you can do that in a straightforward manner. When you build a Unity failover pair, you build the primary server first and one of the latter tasks is to enable failover. At a high-level, you would want to disable failover on the existing primary server then you could go ahead and rebuild a new failover server based on the installation guide procedures. The installation guides for Unity 7 are the same as version 5 so here is a link to the various guides:
http://www.cisco.com/en/US/products/sw/voicesw/ps2237/prod_installation_guides_list.html
You would want to ensure that you install any additional ES releases to match the versioning before systems and ensure you reuse the same service accounts, permissioning, etc. With Unity, the failover license is installed on the failover server so you would need to work out rehosting that license (that is, if you end up replacing the server completely). Once everything is up to spec with the primary server, you would reconfigure the failover wizard. The important thing is that you want the 2 servers to pretty much mirror each other from a build perspective. If your system was built based on the installation recommendations then that shouldn't be much of a complication. You could potentially leverage a DiRT backup to ease the rebuild process but I'm not sure if you'd want to go that route or not. Honestly, it's been a long time since I've looked the documentation and procedures for DiRT. You can find that information at www.ciscounitytools.com if you'd like to look into the details a bit.
With that said, what happens when you attempt to boot the server now? Do you get any error messages to indicate the boot sector and/or Windows image is corrupt? Can you break out of the boot process and boot into safe mode at all? Some more details along those lines would be of interest as well.
Hailey
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